Malignant cell transformation is a multistep process resulting from the progressive acquisition of structural alterations at multiple genetic loci which are involved in the regulation of cell growth. It has been well documented that gain-of-function mutations, found in dominantly-acting proto-oncogenes, are often accompanied by loss-of-function mutations in tumor suppressor genes in human malignant cells. Several tumor suppressor genes have been identified whose mutation or deletion appears to be critical for the development of human cancers, among them, p53, RB and WT1, whose gene products are found in nucleus and which function as regulators of gene transcription (reviewed in Kaelin et al., Cellular proteins that can interact specifically with the retinoblastoma susceptibility gene product. In Origin of human cancer: A comprehensive review, Brugge, J., Curran, T., Harlow, E., McCormik, F. eds. (Cold Spring Harbor Laboratory Press), pp.423-431 (1991); Lewin, Cell 64:303-312 (1991); Marshall, Cell 64:313-326 (1991); Weinberg, Science 254:1138-1146 (1991); Haber and Housman, Adv. Cancer Res. 59:41-68 (1992); Vogelstein and Kinzler, Cell 70:523-526 (1992); Levine, Annu. Rev. Biochem. 62:623-651 (1993)).
Two structurally related transcription factors, IRF-1 and IRF-2 were originally identified as regulators of the interferon (IFN) system (Miyamoto et al., Cell 54:903-913 (1988); Harada et al., Cell 58:729-739 (1989); Tanaka and Taniguchi, Adv. Immunol. 52:263-281 (1992)). IRF-1 has also been identified by others in different contexts (Pine et at., Mol. Cell. Biol. 10:2448-2457 (1990); Yu-Lee et al., Mol. Cell. Biol. 10:3087-3094 (1990); Abdollahi et at., Cell Growth Differ. 2:401-407 (1991); Stark and Kerr, J. Interferon Res. 12:147-151 (1992)). IRF-1 functions as a transcriptional activator whereas IRF-2 represses the effect of IRF-1 by competing for binding to the same DNA sequence elements (IRF-Es) (Harada et al., Cell 58:729-739 (1989); Tanaka et al., Mol. Cell. Biol. 13:4531-4538 (1993)). IRF-Es can be found in both the IFN-.alpha. and IFN-.beta. promoters, as well as in IFN-stimulated response elements (ISREs) found within the promoters of IFN-inducible genes (Friedman and Stark, Nature 314:637-639 (1985); Shirayoshi et al., Mol. Cell. Biol. 7:4542-4548 (1987); Levy et al., Genes Dev. 2:383-393 (1988)). It has been shown that IRF-1 functions as an activator for the type I IFN genes and some IFN-inducible genes (Fujita et al., Nature 337:270-272 (1989); Harada et al., Cell 63:303-312 (1990); Au et al., Nucleic Acids Res. 20:2877-2884 (1992); Pine, J. Virol. 66:4470-4478 (1992); Reis et al., EMBO J. 11:185-193 (1992); Matsuyama et al., Cell 75:83-97 (1993); Ruffner et at., Proc. Natl. Acad. Sci. USA 90:11503-11507 (1993)).
Evidence has also been provided demonstrating a role for IRF-1 as a tumor suppressor; (i) IRF-1 manifests antiproliferative activities (Yamada et al., Proc. Natl. Acad. Sci. USA 88:532-536 (1990); Kirchhoff et al., Nucleic Acids Res. 21:2881-2889 (1993); T. Tamura, M. S. L., and T. Kawakami, unpublished results), (ii) overexpression of the repressor IRF-2 in NIH 3T3 cells causes cell transformation and this cell transformation is suppressed by concomitant overexpression of the activator IRF-1 (Harada et al., Science 259:971-974 (1993)), and (iii) primary embryonic fibroblasts (EFs) with a null mutation in the IRF-1 gene (IRF-1.sup.-/- mice) are susceptible to transformation by an activated form of c-Ha-ras, a property also seen in the EFs from p53.sup.-/- mice, but not in wild type EFs. These observations collectively suggest that the loss of IRF-1 function may contribute to the development of human neoplasia.
The human IRF-1 gene has been mapped to 5q31.1 (Itoh et al., Genomics 10:1097-1099 (1991); Willman et al., Science 259:968-971 (1993); Harada et al., Mol. Cell. Biol. 14:1500-1509 (1994)). Chromosome band 5q31 was previously determined to be the most commonly deleted segment, the so-called "critical region", in human leukemia and MDS with interstitial deletions of chromosome 5q (Le Beau et at., J. Clin. Oncol. 4:325-345 (1986); Nimer and Golde, Blood 70:1705-1712 (1987); Le Beau et al., Cancer Cells 7:53-58 (1989); Pederson and Jensen, Leukemia 5:566-573 (1991)). Del(5q) is also a hallmark of a unique clinical myelodysplastic disorder with refractory anemia and abnormal megakaryocytes occurring predominantly in elderly females, known as the "5q-Syndrome" (Van den Berghe et at., Cancer Genet. Cytogenet. 17:189-256 (1985)). Hence it is believed that this chromosomal region harbors a tumor suppressor gene(s). However, in view of the variable clinical features of myeloid diseases associated with del(5q), it is possible that inactivation of such tumor suppressor gene(s) is accompanied by variable additional genetic events such as the activation of oncogenes in different types of myeloid disorders (Carter et al., Crit. Rev. Oncog. 3:339-364 (1992)).
Previously, it had been demonstrated that one or both IRF-1 alleles were deleted in each of 13 representative cases of MDS and leukemia with del(5q) or translocation of 5q31. Furthermore, inactivating gene rearrangement of one IRF-1 allele, accompanied by deletion of the residual allele, were found in a case of de novo acute leukemia (Willman et at., 1993). These observations support the idea that IRF-1 may be the critical tumor suppressor gene deleted in the del(5q); thus loss of one or both IRF-1 alleles may contribute to unrestrained cellular proliferation thereby promoting the development of human leukemia and MDS. On the other hand, both IRF-1 alleles are still retained in some MDS/leukemia patients exhibiting 5q deletion (Boultwood et al., Blood 82:2611-2616 (1993); Ice Beau et al., Proc. Natl. Acad. Sci. USA 90:5484-5488 (1993)).